Giardia is a disease that comes from cow feces.
And then the cows poo in the Green River, and then your canoe tips, and you aren't expecting it, and you swallow cow feces water and ten days later you get the bloody fuckin' shits.
And you have to go buy Pepto Bismal.
But you still have the bloody shits.
Because Giardia is nasty and Pepto doesn't help.

"I was on the Green River last week, and I accidentally swallowed some water from it, and now I can't leave my bathroom 'cause I got Giardia." said Nicholas,
"Oh so that's why you haven't been at school." said Steve the math teacher.
"Yup, I got the bloody shits."